It is already known from the prior art the injection effected by centrifugation of the aluminum cages in rotors, which are formed by a stack of overlapped annular steel laminations provided with openings that are longitudinally aligned with the openings of the other laminations of the stack, in order to define a plurality of axial channels interconnecting the external faces of the end laminations of the stack and which are angularly spaced from each other along a circular alignment, which is concentric to the longitudinal axis of the lamination stack, but radially spaced back in relation to the lateral face of the latter.
The lamination stack, with its longitudinal axis vertically disposed, is positioned in the interior of a mold that defines a lower annular cavity close to the external face of the lower end lamination, and an upper cavity, which is substantially cylindrical or frusto-conical, close to the external face of the upper end lamination and opened to a channel for the entry of aluminum into the mold.
During the aluminum pouring, the lamination stack has its central axial bore, in which will be later mounted the shaft of the electric motor, filled with a core, which has an upper end substantially leveled with the upper end lamination the lamination loan and stack, and a widened upper end lower portion, which is seated on a respective lower end widening of the central axial hole of the lamination stack and against the mold portion that defines the lower cavity.
The aluminum is poured into the lower cavity, passing through the axial channels of the lamination stack to the lower cavity, filling the latter, the axial channels, and the upper cavity, in this order, and solidifying in a radial inward upward pattern, as the mold rotates around its vertical axis and the metal cools.
Upon completion of the aluminum pouring and solidification, the mold is opened and the formed rotor is submitted to one or more operations to eliminate the inlet channel and unobstruct the adjacent end of the central axial bore of the lamination stack, and to define the correct inner profile for the upper ring of the aluminum cage, which further comprises a single piece lower ring, which is already formed by the mold, and a plurality of bars formed in the interior of the axial channels of the lamination stack.
In the centrifugation injection of these rotors, the upper and lower cavities of the mold and the lamination stack itself are heated, so that the aluminum passes through the upper cavity and through the axial channels of the lamination stack without solidifying, gravitationally reaching the lower cavity, filling it and beginning to solidify, from the outside to the inside and from the bottom upwardly, as the mold rotates.
In order that the injection mold involving and locking superiorly and inferiorly the lamination stack can rotate around its vertical longitudinal axis, the upper and lower cavities of the mold are mounted, respectively, onto an upper bearing and a lower bearing that are carried by the structure of the injection equipment.
In the bearing arrangements of the type mentioned above, the occurrence of deviations of concentricity and parallelism between the axes of the upper and lower cavities cause vibrations in both the mold and the lamination stack during rotation of the mold, which vibrations act on the metallic material being solidified in the upper and lower cavities.
A major problem caused by said vibrations of the rotating mold during the solidification of the aluminum, is that the bars of the cage, which are formed in the interior of the axial channels of the lamination stack, and even the rings tend to present cracks, the bars being transversally broken inside the lamination stack in a way not perceptible by an external visual checking of the finished rotor. The breakage or crack of one or more bars, or of the upper or lower rings of the cage will considerably impair the quality of the rotor and consequently the efficiency of the electric motor to be formed.
One of the possibilities to minimize or even eliminate the loss of quality by undue vibrations of the mold during the aluminum solidification is to mount both cavities of the mold to only one lower bearing, whereby the shafts of both parts of the mold are unified. However, in this solution, the upper and lower cavities of the mold are guided by columns that are affixed to the lower cavity. The upper cavity is axially displaceable, guided by the columns, to open and close the mold, whereby the upper cavity is slidingly retained in the columns, considerably limiting the automation of the operations of feeding the lamination stack in the mold, and also the removal of the centrifuged rotor, besides the problems of concentricity and rotor strike.